(1) Field of the invention
The present invention relates to a system and method for controlling fuel supply to an internal combustion engine according to a predetermined operation of an automatic transmission applicable to an automotive vehicle in which fuel supply cut-off during a predetermined engine deceleration is appropriately carried out.
(2) Background of the art
A Japanese Patent Application First Publication (Unexamined) Showa 62-87641 published on Apr. 22, 1987 and a U.S. Pat. No. 4,387,681 issued on June 14, 1983 exemplify previously proposed fuel supply controlling systems in which fuel supply is halted (hereinafter, referred to as fuel supply cut-off) is carried out when an engine revolutional speed exceeds a predetermined cut-off threshold during deceleration of the engine and the supply of fuel is resumed when the engine revolutional speed is reduced below a recovery speed value so that fuel consumption can be conserved.
In addition, a first control unit used for controlling an engine is installed independently of a second control unit used for controlling a shift range of an automatic transmission. Mutual communication is established between the first and second control units such that the first control unit controls the engine on the basis of an engine control request signal output from the second control unit.
In a previously proposed engine controlling system in which mutual communication between the control units is established, the second control unit temporarily produces an on, off, and on signal derived on the basis of a signal from an idling switch when the automatic transmission executes a shiftdown operation and the first control unit determines whether the fuel supply is to be cut off.
It is noted that when the engine revolutional speed is reduced, the fuel supply cut off is halted with a gear of the automatic transmission temporarily in a neutral position during the shift operation before the above-described operation (refer to FIG. 8). Therefore, in a case where the engine revolutional speed after the shift down operation does not exceed the fuel supply cut off revolutional speed due to a gear ratio, the fuel supply cut off cannot be executed.
If the fuel supply cut off is executed, the cut off may result in a variation of torque so that a shock (or jolt) will be generated.
Furthermore, Japanese Patent Application First Publication (unexamined) Showa 62-159839 published on July 15, 1987, exemplifies an automatic transmission in which a clutch (referred to as an overrun clutch) for controlling engine braking is installed to execute engine braking in accordance with a vehicle running condition in response to a signal derived from the second control unit. Such an overrun clutch as described above is controlled according to the energization and deenergization of an overrun clutch solenoid.
That is to say, when the overrun clutch solenoid is energized, the clutch is disengaged so as not to apply the engine braking force to the vehicle and when the overrun clutch solenoid is deenergized, the clutch is engaged so as to apply the engine braking force.
However, the following problem arises: when the overrun clutch is disengaged, the engine revolutional speed is quickly reduced due to the non-application of engine braking. If the fuel supply is recovered, the engine revolutional speed often drops to a speed below a normal idling speed. Consequently, engine stalling occurs.
On the other hand, if the fuel recovery speed is increased, the fuel recovery timing becomes earlier than expected so that the fuel consumption cannot be conserved.